When we listen to someone or something, interference noise or undesired acoustic signals are everywhere present that interfere with the voice of someone opposite us or with a desired acoustic signal. People with a hearing impairment are especially susceptible to such interference noise. Background conversations, acoustic disturbance from digital devices (cell phones), or noise from automobiles or other ambient sources can make it very difficult for a hearing-impaired person to understand a wanted speaker. A reduction of the noise level in an acoustic signal coupled with an automatic focusing on a desired acoustic signal component can significantly improve the efficiency of an electronic speech processor of the type used in modern hearing aids.
Hearing aids have very recently been introduced that employ digital signal processing. They contain one or more microphones, A/D converters, digital signal processors, and loudspeakers. The digital signal processors usually divide the incoming signals into a plurality of frequency bands. An amplification and processing of signals can be individually adjusted within each band in keeping with requirements for a specific wearer of the hearing aid in order to improve a specific component's intelligibility. Further available in connection with digital signal processing are algorithms for minimizing feedback and interference noise, although they have significant disadvantages. What is disadvantageous about the currently employed algorithms for minimizing interference noise is, for example, the maximum improvement they can achieve in hearing-aid acoustics when speech and background noise are located within the same frequency region, which renders them incapable of distinguishing between spoken language and background noise. (See also EP 1 017 253 A2)
That is one of the most frequently occurring problems in acoustic signal processing, namely filtering out one or more acoustic signals from among different such signals that overlap. The problem is referred to also as what is termed the “cocktail party problem”. All manner of different sounds including music and conversations therein merge into an indefinable acoustic backdrop. People nevertheless generally do not find it difficult to hold a conversation in such a situation. It is therefore desirable for hearing-aid wearers to be able to converse in just such situations like people without a hearing impairment.
Within acoustic signal processing there exist spatial (directional microphone, beam forming, for instance), statistical (blind source separation, for instance), and hybrid methods which, by means of algorithms and otherwise, are able to separate out one or more sound sources from among a plurality of simultaneously active such sources. Thus by means of statistical signal processing performed on at least two microphone signals, blind source separation enables source signals to be separated without prior knowledge of their geometric arrangement. When applied to hearing aids, that method has advantages over conventional approaches based on a directional microphone. With said type of BSS (Blind Source Separation) method it is inherently possible with n microphones to separate up to n sources, meaning to generate n output signals.
Known from the relevant literature are blind source separation methods wherein sound sources are analyzed by analyzing at least two microphone signals. A method of said type and a corresponding device therefore are known from EP 1 017 253 A2, the scope of whose disclosure is expressly to be included in the present specification. Relevant links from the invention to EP 1 017 253 A2 are indicated chiefly at the end of the present specification.
In a specific application for blind source separation in hearing aids, that requires two hearing devices to communicate (analyzing of at least two microphone signals (right/left)) and both hearing devices' signals to be evaluated preferably binaurally, which is performed preferably wirelessly. Alternative couplings of the two hearing devices are also possible in an application of said type. A binaural evaluating of said kind with a provisioning of stereo signals for a hearing-aid wearer is disclosed in EP 1 655 998 A2, the scope of whose disclosure is likewise to be included in the present specification. Relevant links from the invention to EP 1 655 998 A2 are indicated at the end of the present specification.
The controlling of directional microphones for performing a blind source separation is subject to equivocality once a plurality of competing useful sources, for example speakers, are presented simultaneously. While blind source separation basically allows the different sources to be separated, provided they are spatially separate, the potential benefit of a directional microphone is reduced by said equivocality, although a directional microphone can be of great benefit in improving speech intelligibility specifically in such scenarios.